What structure in plants is responsible for adding rigidity?

The xylem is the correct answer because it plays a critical role in providing structural support and rigidity to plants. Xylem tissue is composed of various types of cells, including tracheids and vessel elements, which are specialized for transporting water and minerals from the roots to other parts of the plant. This water transport is vital, but the xylem also contributes to the plant's overall strength. The walls of xylem cells are thickened and often lignified, which means they contain lignin, a complex organic polymer that adds rigidity and firmness to the plant structure.

In contrast, phloem primarily focuses on the transport of sugars and nutrients produced through photosynthesis but does not contribute significantly to rigidity. Pith is involved in storing nutrients and may provide some support, but its role is more about storage than structural integrity. Similarly, the cortex, which lies just beneath the epidermis, is mainly involved in storage and transport but does not serve the primary function of providing rigidity like xylem does. Therefore, the significant structural contribution of xylem makes it the key tissue responsible for adding rigidity to plants.